1. Field
An electromagnetic coil for an inductive sensor is disclosed, such as a coil of the type that is used in sensors that operate according to the principle of electromagnetic force compensation, wherein the coil is provided with protective means against the penetration of moisture. A force-measuring cell with this type of coil and a method of applying a protective coating to a coil are also disclosed.
2. Background Information
An inductive sensor is based for example on the concept of a coil that is movable in an inhomogeneous magnetic field, where a current is induced in the coil when the latter moves in a direction of the inhomogeneity. The induced current represents a measure for the displacement of the coil and can be used to measure a force that is causing the displacement.
In other inductive sensors of the type used with preference in force-measuring cells, a current flows through a coil that is movable in the homogeneous magnetic field of a permanent magnet. A force acting on the coil causes a displacement of the coil which is detected by an optical position sensor, whereupon a servo-control circuit changes the flow of current in the coil to the magnitude required to hold the coil in its original position again. The change in the current is commensurate with the force that is to be measured by the force-measuring cell. This measurement principle is referred to as electromagnetic force compensation.
A force-measuring device that operates according to the foregoing principle of electromagnetic force-compensation, which finds application for example in the field of weighing technology, includes a force-transfer device with a parallel-guiding mechanism and in many cases a lever mechanism for the reduction of a force, e.g. a load, to be transmitted. The sensor has a permanent magnet with an air gap, with a coil being immersed in the magnetic field in the air gap of the permanent magnet. If the force-measuring device has a lever mechanism, the coil can be arranged at the longer lever arm of the last lever. A coil of the type used here has one or more windings of an insulated metal wire, normally a copper wire. The electrical insulation of the wire is required in order to avoid electrical contact between adjacent windings.
The coil is in many cases wound on a coil former which lends the necessary degree of stability to the windings. However, there are also so-called air coils whose shape is maintained by an adhesive compound connecting the windings to each other.
Force-measuring cells that work according to the principle of electromagnetic force compensation, in particular if they are designed for a high measurement resolution, often have to meet the requirement of a high sensitivity that does not change, for example during the operation of a balance that is equipped with the force-measuring cell.
Besides other parameters not specifically mentioned here, the coil is also a contributing factor in a sensitivity change of the force-measuring cell, which is due in particular to moisture absorption of the insulating material that sheathes the coil wire. Furthermore, moisture absorption of the wire insulation causes a shift of the zero point of the force-measuring cell. The insulation of the coil wire consists in almost all cases of a polymer, for example of polyurethane, polyimide, or polyamidimide.
Moisture absorption of the insulating material of the coil wire, or generally a change in the moisture content of the insulating material which depends on the ambient humidity of the force-measuring cell, can lead for example to leakage currents between the windings. As another possible consequence of a moisture absorption or moisture release, the insulating material can swell up or shrink. The result is in both cases a geometric change of the coil, particularly of the spacing between neighboring windings, which will also lead to mechanical stress inside the coil. The change in the moisture content is further accompanied by a weight change of the coil.
All of these moisture-related effects can, overtime, cause changes in the measuring result, if this type of coil is used in an inductive measuring sensor. In particular in the case of a force-measuring cell that operates according to the principle of electromagnetic force compensation, the moisture absorption of the insulating material of the coil wire is one of the factors that can determine the sensitivity of the measuring cell.
The existing state of the art includes a variety of measures that have been proposed or are used in practice to avoid or at least reduce moisture absorption in a coil.
As an example, a ring-shaped sleeve of aluminum is shrink-fitted on a coil that is wound on a coil former. Due to the thermal expansion that occurs when the sleeve is heated up, it can be fitted over the coil former on which the coil has been wound, and in the subsequent cooling, the aluminum sleeve contracts itself snugly against the coil and the coil former, whereby a certain sealing effect is achieved against an exchange of moisture with the environment. In some cases, if a higher degree of seal-tightness is required, the sleeve is welded to the coil former, leaving only a passage opening for the conductor leads to the coil, which requires additional sealing by means of a lacquer.
In the effort to improve the measuring results in an environment with relatively high humidity fluctuations, for example between 20% and 80% relative air humidity, satisfactory results—at least in force-measuring cells with relatively low sensitivity requirements—are also achieved with a thick lacquer coating applied directly to a coil and solidified by a thermal treatment.
A watertight encapsulation of the coil component of the electromagnetic unit of an electromagnetic balance is disclosed in JP 25 18171 Y2, the disclosure of which is hereby incorporated by reference in its entirety. The coil is arranged on a coil former that is configured in such a way that the coil windings lie in a recessed part of the coil former. The coil is enclosed inside the recessed space of the coil former by means of a watertight ring sleeve that is fitted to the coil former. A tightly holding snap connection exists between the coil former and the watertight ring sleeve.
Ring sleeves used as sealing enclosures have the drawback that the overall weight of the coil is increased, but their most decisive disadvantage is that the geometric dimension of the coil, particularly its thickness, is strongly enlarged, so that the width of the air gap of the permanent magnet has to be adapted accordingly. However, enlarging the air gap of a magnet system that is otherwise unchanged decreases the magnetic field in the air gap, which has a negative effect on the sensitivity of the sensor. If the same sensitivity is to be maintained with an enlarged air gap, a larger magnet system will be required which will, however, be more difficult to handle and more expensive to manufacture.
Furthermore, coils that are sealed in accordance with the existing state of the art still require special sealing measures at the passage opening for the conductor leads to the coil. The aforementioned lacquer seal does not provide the necessary seal-tightness to achieve the low moisture-absorption values that are required for a high sensitivity in the case of force-measuring cells with a high measurement resolution. The same can also be said to the aforementioned method of sealing the entire coil with a lacquer coating.